1. Field of the Invention
The present invention is related to an electrical testing device and its testing method, and more particularly, to a low-pass filter with automatically switching function used in the multi-meter.
2. Description of the Prior Art
The conventional low frequency signal is often transmitted with noise. For example, when testing mains power, the frequency of mains power is mostly low frequency signal at 50 Hz or 60 Hz, which is itself easy to be interrupted and often transmitted with noise. The noise will affect the evaluating result of the true RMS and cause error of voltage measuring value. In addition, take an advanced multi-meter for example, AC voltage can be measured at the same time when the frequency of the signal is evaluated. However, if the amplitude of the noise is too large, the Schmitt trigger will generate too many pulses, and the frequency evaluating result will be too high.
Therefore, in order to filter the high frequency noise, a low-pass filter is usually used for filtering the high frequency noise. But when a low-pass filter is used, if the input signal is itself a high frequency signal, the amplitude of the input signal will be attenuated, or even completely filtered, by the low-pass filter. In other words, when the frequency of input signal itself is as high as being close to the −3 db frequency of the low-pass filter, the amplitude of the filtered signal will start attenuating. And the higher the frequency, the more serious the attenuating problem is. Therefore, if the low-pass filter is directly utilized in the testing device for filtering, the low-pass filter will efficiently filter out the noise in the low frequency signal; however, when the testing signal is a high frequency signal (i.e. not a high frequency noise), the attenuation of the amplitude of the high frequency signal will cause lower testing value and thus mislead the user. For example, if the −3 db frequency of the low-pass filter is 1 KHz and the testing signal at the input end is a 10 KHz/300 V high frequency signal, a signal with frequency obviously higher than the −3 db frequency of the low-pass filter, the amplitude of the signal will be attenuated and the testing value will be very low and may lead to disregard of user and thus risk of electric injury. Moreover, take the advanced multi-meter for example, usually when the voltage value or the current value of the testing signal is detected as larger than the preset value, the buzzer of the multi-meter will buzz to alarm the user. Yet if the low-pass filter is operating in the multi-meter and causing the testing result much lower than its real value, the buzzer will lose its alarming function. In addition, if the attenuation of the signal is too serious, the Schmitt trigger is not able to generate pulses and the frequency is not able to be evaluated normally.
A method of manually activating low-pass filter is disclosed in U.S. Pat. No. 7,034,517 B2. The user has to determine whether to turn on the low-pass filter or not and push the button on the testing device to turn it on. However, when the user turns on the low-pass filter and the frequency of the testing signal becomes high frequency later on, error of testing result may also happen without being noticed by the user.
In order to filter the high frequency noise and maintain the accuracy of the testing result of the high frequency signal at the same time, an automatically switching function is disclosed in the present invention. The low-pass filter can be switched on and off according to different status of the signal at the input end to enhance the accuracy and the expediency of testing.
In the initial status, the low-pass filter is activated and the switches are used to select modes for evaluation within the testing device. When the frequency of the input signal is detected after operation to be close to or higher than the −3 db frequency of the low-pass filter, the −3 db frequency of the low-pass filter will automatically increase. If the input signal is detected as a high frequency signal, the low-pass filter will automatically turn off and the signal without being filtered is selected to be evaluated, and thus the accuracy and the expediency of testing will be enhanced.